1. Field of the Invention
The present invention relates to an image pickup apparatus for picking up an object image.
2. Related Background Art
In recent years, digitization has made progress in various fields of medical care. In a field of X-ray diagnosis, a two-dimensional X-ray image pickup apparatus for converting incident X-rays to visible light by a scintillator (phosphor) and further picking up an image of such visible light by an image pickup element has been developed for digitization of images.
As the two-dimensional X-ray image pickup apparatus, for example, a compact CCD type image pickup element has been put in practical use for dental use and a large area still image pickup apparatus which uses a maximum 43 cm×43 cm amorphous silicon (a-Si) has also been put in practical use for photographing breasts and chests. In an image pickup element using an amorphous silicon semiconductor on a glass substrate, it is easy to manufacture one with a large area. Some large area X-ray image pickup apparatuses are realized by tessellating four panels of the image pickup element. As an example of this type of technology, there is an apparatus described in U.S. Pat. No. 5,315,101.
In addition, it has been proposed to constitute a large area X-ray image pickup apparatus using a plurality of monocrystal image-pickup elements (silicon image pickup element, etc.). As examples of this type of technology, there are apparatuses described in U.S. Pat. No. 4,323,925 and U.S. Pat. No. 6,005,911. As the monocrystal image pickup element, there are, for example, a CCD type image pickup element and MOS and CMOS type image pickup elements, etc. using silicon. In a field of medical X-ray diagnosis in which digitization has made progress as described above, it is expected that a moving image pickup apparatus (fluoroscopy, etc.) of the next generation of the still image pickup apparatus will be realized.
As technical objects in this field, there are: (1) high sensitivity and high-speed reading technology; (2) increase in size; (3) reduction of costs, and the like. Concerning the objects of high sensitivity and high-speed reading, it is required, for picking up a moving image, to realize high sensitivity and a reading speed that are more than ten times as high as those of an image pickup apparatus using an amorphous silicon. In order to pick up a moving image, X-rays are continuously irradiated on a human. Thus, taking into accounts effects caused by X-ray irradiation, an irradiation amount is required to be reduced to one several tenth to one hundredth and a reading speed is required to be increased to 60 to 90 frames/second. In order to carry out reading at such a speed, sensitivity and high speed several ten times as high as those of the image pickup apparatus using the amorphous silicon are required.
The amorphous silicon does not have a sufficient semiconductor characteristic with respect to a high-speed operation. With a large area image pickup apparatus using the amorphous silicon, micro-machining of a semiconductor on a glass substrate is more difficult compared with a monocrystal silicon semiconductor substrate. As a result, a capacitance of an output signal line increases. This capacitance becomes a largest cause of a noise (kTC noise). A manufacturing process of an amorphous silicon large plate image pickup apparatus is more advantageous in that an element with a larger area as compared with the CCD type image pickup element and the CMOS type image pickup element is obtained. However, its photoelectric conversion portion is not a full depletion type, a driving circuit and an amplifier of an image pickup element are required on the outside (see FIG. 52 of Japanese Patent Application Laid-open No. 8-116004) and judgement of undefective products is required to be carried out after incorporating peripheral parts. Therefore, while the image pickup element itself is relatively low-priced, costs are ultimately high. Due to these reasons, it is difficult to satisfy the above-mentioned requirements.
In addition, the CCD type image pickup element is the full depletion type and has high sensitivity but is unsuitable as a large area image pickup apparatus. The CCD type image pickup element is a charge transfer type, and thus the larger it becomes and as the number of transfer steps increases (higher number of pixels), the more problematic the transfer becomes. That is, driving voltages are different at a driving end and around the center, which makes complete transfer difficult. In addition, power consumption is represented by CVf2 (C is a capacitance between a substrate and a well, V is a pulse amplitude and f is a pulse frequency). The larger an element is, the larger C and V become, whereby the power consumption becomes ten or more times larger than that of the CMOS type image pickup element. As a result, the driving circuit in its periphery becomes a heating source and a noise source and a high S/N is not obtained. Thus, the CCD type image pickup element is not suitable for a large image pickup element in some aspects.
Moreover, with a structure of a simple large area image pickup apparatus using a plurality of monocrystal image pickup elements, a dead space is inevitably generated in a joining portion of each image pickup element (because an area for providing an external terminal, which exchanges signals and power source with peripheral circuits such as a shift register and an amplifier or with the outside, and a guard circuit is always necessary, separately from areas for the image pickup elements). This portion becomes a line defect and decreases an image quality. Thus, a structure is employed which uses a taper-shaped FOP (fiber optic plate) to guide light from a scintillator to the image pickup elements avoiding a dead space. However, a redundant FOP is necessary and manufacturing costs increase. In particular, the taper-shaped FOP is extremely costly. Moreover, light from the scintillator is less easily incident on the FOP according to a taper angle of the taper-shaped FOP and decrease of an output light amount occurs, which offsets sensitivity of the image pickup elements to deteriorate sensitivity of the entire apparatus.
In order to compensate for the above-mentioned drawbacks of the amorphous silicon image pickup element and the CCD-type image pickup element, a structure in which large area CMOS type image pickup elements are tessellated is proposed (Japanese Patent Application Laid-Open No. 2000-184282).
However, a conventional amplification-type image pickup element such as the CMOS type image pickup element has the following inconveniences.
(a) With a general driving method of the amplification-type image pickup element, accumulated electric charges for one horizontal scanning line are sequentially read out with a horizontal scanning line of an identical column as a unit. While the accumulated electric charges are read out from a certain horizontal scanning line, electric charges are accumulated in the remaining horizontal scanning lines. In this case, time for accumulating electric charges is different for each horizontal scanning line. If the electric charges are read out and reproduced as an image, images of different timing for each scanning period are obtained. In photographing of a still image, it is less likely that this difference in the accumulation time becomes a problem. However, in photographing of a moving image, an image is smeared to cause a problem. In particular, in an image pickup apparatus with a plurality of image pickup elements tessellated (an image pickup element panel in which a plurality of pixels are formed), noncontinuity occurs in an image between each image pickup element to cause a significant problem as discussed below. In addition, with X-ray moving image photographing, reading-out time of a certain horizontal scanning line coincides with exposure time of the other horizontal scanning lines and unnecessary X-ray irradiation must be performed partially. Therefore, it is difficult to apply this method to a medical field in which an amount of exposure to radiation should be reduced as much as possible.
(b) There is a method of fixing an accumulation period of electric charges in each horizontal scanning line by providing a mechanical shutter in order to prevent accumulation periods of electric charges from being different in a horizontal scanning line for reading out first and a horizontal scanning line for reading out later. However, this method has a disadvantage that an apparatus is enlarged in size.
Problems in the case where high-speed moving image photographing is performed using an image pickup apparatus in which four large area CMOS type image pickup elements described above are tessellated, in particular, problems relating to the above item (a) will be described below. FIG. 1 shows a plan view of an image pickup apparatus in which four image pickup elements are tessellated. Image pickup areas (image pickup element panels) A1, A2, B1 and B2 are constituted by arranging a plurality of pixel portions in horizontal and vertical directions. Reference symbol Hn in the image pickup areas denotes rows that are scanned by row scanning circuits and Vn denotes columns that are scanned by column scanning circuits. In addition, a row scanning circuit, a column scanning circuit, a memory circuit and an output amplifier are provided for each image pickup area.
FIG. 2 shows a schematic structure of one pixel portion and a signal reading-out circuit of each image pickup element. In FIG. 2, a method for scanning each column to read out a signal is employed. In addition, in a conventional circuit of FIG. 2, a signal reading-out circuit is a dual sampling circuit as described in detail later. In FIG. 2, reference symbol VSR denotes a column scanning circuit and HSR denotes a row scanning circuit. In addition, reference symbol PD denotes a photodiode, TR1 denotes a transfer switch, TR2 denotes a reset switch, TR3 denotes a column selection switch, TR4 denotes an amplification transistor, TR5 denotes a switch for resetting a signal line, TR6 and TR7 denote sample switches and TR8 and TR9 denote reading-out switches. Reference symbols TR1 to TR9 denote MOS transistors. In addition, reference symbol CTS denotes an optical signal holding capacitor and CTN denotes a reset signal holding capacitor.
In the conventional circuit of FIG. 2, a reset signal (a noise component and a dark current component) is held in the reset signal holding capacitor CTN and an optical signal (an optical signal component, a noise component and a dark current component) is held in the optical signal holding capacitor CTS as described in detail later. Thereafter, signals held in the respective holding capacitors CTN and CTS are read out to detect a difference by a differential circuit (not shown), whereby an optical signal with the noise component removed is outputted. In an image pickup apparatus in which a plurality of such image pickup elements are tessellated together, “binding or seam” of moving images between image pickup elements is important when an image of a moving object is picked up.
FIG. 3 shows image composition in the case where the four image pickup elements are tessellated together. When the four image pickup elements are independently driven in a scanning direction as shown by arrows in FIG. 3, correlation of images at binding portions of four images (a connecting portion of image pickup areas A1 and B2, a connecting portion of the image pickup areas B1 and A2, a connecting portion of the image pickup areas A1 and B1 and a connecting portion of the image pickup areas B2 and A2) is eliminated. For example, temporal deviation for a scanning period in a column direction occurs between a column of the image pickup area A1 (a column where scanning ends) and a column of the image pickup area B2 (a column where scanning starts) that are adjacent with each other in the vicinity of the connecting portion of the image pickup areas A1 and B2, and thus correlation of images is lost. At this point, “binding” of moving images are concerned basically in portions of the image pickup areas A1 and B1, the image pickup areas A1 and B2, the image pickup areas B2 and A2 and the image pickup areas B1 and A2 where the images are connected. With a structure in which the image pickup elements using the amplification-type image pickup element such as the CMOS type image pickup element are tessellated together in this way, correlation of images is lost at bindings between the image pickup elements, resulting in decrease in image quality.